In spinal fusion, two or more vertebrae are joined by additional bone material placed between the vertebrae. Once fusion is complete, the bone material immobilizes the vertebrae. Spinal fusion is used primarily to treat pain caused by abnormal motion of the vertebrae. Anterior lumbar interbody fusion (ALIF) is a spinal fusion technique that can be used for treating degenerative discs from an anterior approach. The anterior approach allows access to the interbody space with minimal damage to the posterior musculature, while allowing full decompression of the diseased disc. During an ALIF procedure, an interbody device is inserted within the intervertebral body space. This interbody is generally composed of PEEK or titanium with a central opening for bone graft material, which is typically an autograft or allograft material. The objective of interbody fusion is to fuse the central graft material to the cranial and caudal endplates, creating a rigid boney union between motion segments.
Known interbody designs have a propensity to stress-shield the graft material. That is, the interbodies, or the fasteners used to anchor the interbodies, absorb axial loads during settling of the implant. This has the effect of shielding the graft material from axial loads. Some interbody designs are configured to expand in an axial direction after being implanted to increase the height of the disc space to a desired spacing. This also stress-shields the graft material, and actually removes load from the graft material because the height of graft space expands.